This invention relates to the evaluation of the dynaminc, mechanical properties of materials. More particularly, the invention relates to a device for, and a method of, evaluating the dynamic, mechanical properties of materials. The invention has particular, but not necessarily exclusive, application in the measurement of the dynamic, mechanical properties of materials in the medical field such as, for example, articular cartilage, other tissue, synovial fluid, or the like.
Articular cartilage is a connective tissue, which covers the ends of bones in synovial joints of humans and animals. It is relatively softxe2x80x94about 1000 times less stiff than bone with a smooth whitish appearance in young joints and a rough yellowish appearance in old joints. Articular cartilage has a number of functions which include, 1) minimising contact stress by distributing joint loading, 2) dissipating some of the energy associated with load bearing and 3) permitting near frictionless motion between articulating surfaces.
Diseases (such as arthritis) often lead to joint inflammation and damage to articular cartilage resulting in the need for surgical intervention. Alleviation of pain and disability may involve medical treatment or surgical excision of damaged or degenerate cartilage. In the operating theatre the choice of procedure is often based on arthroscopic examination of the cartilage surfaces. This method is sometimes insecure and does not give any indication of sub-surface degradation. Early diagnosis of osteoarthritis is essential to avoid progression to the clinical stage as once erosion has occurred the disease is irreversible. In nearly all cases of symptomatic patients presenting with visually normal cartilage, the cartilage is found to be softened when probed. Indeed softening of cartilage is one of the earliest observable changes caused by osteoarthritis.
A significant amount of research has been carried out in vitro on the mechanical properties of cartilage using bench top materials testing apparatus. A number of measuring instruments for assessing the mechanical properties of cartilage in-vivo have been proposed (see, for example U.S. Pat. No. 4,364,399, U.S. Pat. No. 5,494,045, U.S. Pat. No. 4,132,224, U.S. Pat. No. 5,503,162). In general, each of these devices measures the surface stiffness of cartilage by means of a probe, which depresses the cartilage locally and from the force resisting penetration determines the local static stiffness. While this method has been found to provide more information about sub-surface degradation it does not fully characterise the properties of the cartilage, which must respond to dynamic loading. This invention provides for dynamic evaluation of the cartilage by causing the probe to depress the surface of the cartilage in an oscillatory manner.
According to a first aspect of the invention, there is provided a device for evaluating dynamic, mechanical properties of materials, the device including
a housing;
a resiliently flexible beam displaceably arranged relative to the housing so as to be displaceable in a reciprocatory manner relative to the housing upon the application of an electric field, the beam comprising a bimorph being a laminate of at least two piezoelectric materials;
an engaging means carried by the beam for engaging the material to be evaluated; and
a deflection measuring means arranged on the beam for measuring displacement of the beam.
Preferably, the piezoelectric materials are piezoelectric ceramic materials.
The manner in which the beam is attached to the housing will depend on the application of the device. In one embodiment of the invention, the beam may be arranged in a cantilevered fashion to extend from a support in the housing with the engaging means, which may be in the form of a probe, arranged at a free end of the beam.
The engaging means may extend from the beam. Once again, depending on the application of the device, the engaging means may extend at an angle to the beam or may be in line with the beam. In the latter case, the engaging means may project from an end of the beam.
In another embodiment of the invention, the beam may be supported at both ends to be able to flex in its central region under the application of the electric field. The engaging means may then be arranged in the central region of the beam.
The electric field applied to the beam may be in the form of a voltage arising from a voltage generating means which generates an AC signal of the appropriate frequency or frequencies.
The deflection measuring means may be in the form of a strain gauge array attached to the beam. Where the beam is mounted in a cantilevered fashion, the array may be arranged at, or adjacent, the mounted end of the beam. In the case where the beam is supported at both ends, the array may be arranged at, or adjacent, one or both ends of the beam.
In the case where the device has the beam mounted in a cantilevered fashion, the device may include a plug received in the housing. One end of the plug may serve as an anchor for the beam. An opposed end of the plug may have an electrical connecting means for connection to a complementary electrical connector.
In the case where the beam is supported at both ends, the housing may include support members extending from a floor of the housing on which the beam rests.
The device may include a processing means connectable to the deflection measuring means for assessing the dynamic behaviour of the beam, in use, to evaluate the dynamic, mechanical properties of the material.
In one embodiment, the processing means may be operable to determine a first signal with the engaging means out of contact with the material and at least one further signal with the engaging means in contact with the material and a combining means for combining the signals so as to separate the influence of the beam and the engaging means from the material being evaluated.
In use, the voltage generating means which generates the oscillating electric field may generate a complex waveform signal to enable the dynamic properties of the material to be determined over a range of frequencies.
In another embodiment, the processing means may incorporate an electronic unit for monitoring a force resisting penetration or displacement of the material by the engaging means and the motion of the engaging means. This electronic unit may include a load-sensing element in the form of a piezoelectric load cell and associated electronic circuitry. The associated electronic circuitry may incorporate a field effect transistor.
The electronic unit may be interposed between the engaging means and the beam and may be sealed in a fluid-impervious casing.
According to a second aspect of the invention, there is provided a method of evaluating dynamic, mechanical properties of materials, the method including the steps of
urging an engaging means into contact with a material to be evaluated, the engaging means being carried on a resiliently flexible beam responsive to an electric field, the beam being a bimorph comprising a laminate of at least two piezoelectric materials;
applying an oscillating electric field to the beam to cause it to oscillate at a predetermined frequency so that the engaging means periodically applies a deforming force to the material;
monitoring the dynamic behaviour of the beam as it oscillates; and
extracting from the monitored, dynamic behaviour of the beam, data relating to the dynamic, mechanical properties of the material.
The method may include exciting the beam with a complex waveform signal.
The method may include using data generated by the oscillation of the beam under the influence of said signal to determine the dynamic, mechanical properties of the material at different frequencies.
The method may include obtaining a first signal with the engaging means out of contact with the material and at least one further signal with the engaging means in contact with the material and combining the signals so as to separate the influence of the engaging means from the material being evaluated.
Thus, the method may include using the signals to form transfer functions. Finally, the method may include processing the transfer functions to simultaneously give values of the dynamic properties of the material at a range of frequencies.